1. Field of the Invention
The invention relates to an activation unit for a motor vehicle brake system of the “brake-by-wire” type, which has:                a) a brake booster which can be activated either by means of an input element which can be brought into a force-transmitting connection with a brake pedal or as a function of a driver's request or independently of the driver's will by means of an electronic control unit, with means which permit decoupling of a force-transmitting connection between the brake pedal and brake booster in the “brake-by-wire” operating mode being provided between the brake pedal and the brake booster,        b) a master brake cylinder which is connected downstream of the brake booster,        c) means for sensing a driver's request or the activation travel of the brake pedal,        d) a pedal travel simulator which interacts with the brake pedal and by means of which a restoring force which acts on the brake pedal In the “brake-by-wire” operating mode can be simulated independently of activation of the brake booster, and        e) a hydraulic activation and deactivation device which activates the pedal travel simulator in the “brake-by-wire” operating mode and deactivates it outside the “brake-by-wire” operating mode, and which is formed by a cylinder-piston arrangement whose pressure space is connected by means of a disconnectable connection to a pressure medium volume accommodating element and on whose piston a simulator housing is supported.        
2. Description of the Related Art
Such an activation unit is known, for example, from the applicant's international patent application WO 2006/084864 A1. The support of the abovementioned simulator housing on the activation and deactivation device is provided in the abovementioned activation unit by means of a piston rod via which the movement of the brake pedal or of the simulator housing is transmitted to the piston outside the “brake-by-wire” operating mode. The piston rod is rigidly connected to the piston and guided by it. The piston has, at its end facing away from the pressure space, an annular face which, in the unactivated position of the arrangement, bears against a metallic stop ring. However, it is felt to be disadvantageous that when the brake pedal is released suddenly high acceleration values occur at the stop, and said high acceleration values may cause undesired play between the piston rod and the piston of the cylinder-piston arrangement.